Mystifying misfires on a 99 Tahoe

[Get Bent]

Right now I have the cable and software from getting my PCM reprogrammed. I think I might look at the knock retard and knock counts and see if I can determine anything from that information. If anyone has other ideas on likely sensors to look at, feel free to suggest them. I am unable to post the files because of the extension. Does anyone know how I can do this? I am using EFILive.

 

[dmad1]

I had a cylinder on an older one and dealer wanted to do a valve job on it. Took it to an independent and described the problem. He raised the hood, pulled out his pocket knife and removed a vacuum line that connected to the intake manifold and cut about an inch off of it. The end of the hose was split just enough to let it suck air and loose vacuum. No charge. Just come back to see us...

 

[Get Bent]

I had a cylinder on an older one and dealer wanted to do a valve job on it. Took it to an independent and described the problem. He raised the hood, pulled out his pocket knife and removed a vacuum line that connected to the intake manifold and cut about an inch off of it. The end of the hose was split just enough to let it suck air and loose vacuum. No charge. Just come back to see us...

Was it the line coming off of the brake booster? I have removed the line going to the heater control valve and the valve itself. This valve costed me an engine. I think the only vacuum lines I have are the EVAP line coming off of the purge valve, the PCV valve and the main vacuum line coming off of the booster. I have ruled out the EVAP line. I unplugged it and capped it off with no change. I think I'm going to change the plugs again and see what happens.

 

[east302]

The heater valve solenoid (on the firewall) is energized when the dash temp dial is set to the “max” detent. That closes the heater valve using vacuum from the line that connects on the PCV valve fitting. If it’s not used, best bet is to just unplug the solenoid.

Since it’s winter, you probably don’t have it set to max cold and probably wouldn’t see a vacuum loss there.


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[Get Bent]

FIXED: jerking on acceleration.
I replaced the MAP sensor with an ACDelco unit
Thanks to everyone who have shared their experience in trying to diagnose this issue.​

 

[lowpost99]

FIXED: jerking on acceleration.
I replaced the MAP sensor with an ACDelco unit
Thanks to everyone who have shared their experience in trying to diagnose this issue.​

Was your truck jerking at all acceleration speeds or like 45-55mph gone when you powered past those speeds? Where is the map sensor reside?